See more information about "Strata" Smith and his original geologic map of England.
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For each dating or chronological method there is a link in the box at right to take you to that section of this page.
There, you will find a brief description of the method, plus links to take you to other webpages with more extensive information.
This is made up of numerous regional tree-ring chronologies, particularly in the medieval and post-medieval periods, for which the laboratory now has more than 200 reference chronologies from many areas. By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.
There are over 130 radiocarbon dating laboratories around the world producing radiocarbon assays for the scientific community. A series of movie clips walks you through the process.
Cross-dating is a technique used to take advantage of consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative chronology.
A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy.
These tests have consistently given the same ages for each of these objects.
Examples of a number of consistent dates derived from different methods are given.
Keyed to the relative time scale are examples of index fossils, the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved.
William "Strata" Smith, a civil engineer and surveyor, was well acquainted with areas in southern England where "limestone and shales are layered like slices of bread and butter." His hobby of collecting and cataloging fossil shells from these rocks led to the discovery that certain layers contained fossils unlike those in other layers.
Trees from the same species, growing in the same area or environment will be exposed to the same conditions, and hence their growth rings will match at the point where their lifecycles overlap.